Synthesis and reactivity of cobalt complexes derived from tris(2-pyridylthio)methane ligand: Structural characterization of cobalt(III) complexes containing cobalt–carbon bond

1394-1402The synthesis, characterization and reactivity of a cobalt(II) complex, [(HL1)CoII(PyS)](ClO4) (1) (where HL1 = tris(2-pyridylthio)methane and PyS = monoanionic pyridine-2-thiolate) are discussed. Complex (1) reacts with molecular oxygen to yield a mononuclear low-spin cobalt(III) complex, [(L1)CoIII(PyS)](ClO4) (2). On the other hand, treatment of (1) with a protic acid (pyridinium perchlorate) followed by a base (triethylamine) and dioxygen forms an isomeric cobalt(III) complex, [(L2)CoIII(PyS)](ClO4) (3) (L2 = 1-[bis(2-pyridylthio)methyl]pyridine-2-thione). Ligand HL1 (in 1) rearranges to L2 (in 3) during the reaction as a result of C–S bond cleavage and subsequent C–N bond formation. X-ray crystal structures of both (2) and (3) reveal a distorted octahedral coordination geometry at cobalt(III) center with a strong cobalt–carbon bonding interaction. A four-coordinate distorted tetrahedral cobalt(II) complex, [CoII(PySH)4](ClO4)2 (4) is formed via C–S bond cleavage of HL1 in the reaction of (1) with an excess amount of pyridinium perchlorate. The electronic structure of (2) as established by DFT calculation suggests a delocalized LUMO with significant contribution from the metal ion. The organocobalt(III) complex converts to an air-stable organocobalt(II) complex (2red) upon one-electron reduction

Oxygenation of Organoboronic Acids by a Nonheme Iron(II) Complex: Mimicking Boronic Acid Monooxygenase Activity

Phenolic compounds are important intermediates in the bacterial biodegradation of aromatic compounds in the soil. An <i>Arthrobacter sp.</i> strain has been shown to exhibit boronic acid monooxygenase activity through the conversion of different substituted phenylboronic acids to the corresponding phenols using dioxygen. While a number of methods have been reported to cleave the C–B bonds of organoboronic acids, there is no report on biomimetic iron complex exhibiting this activity using dioxygen as the oxidant. In that direction, we have investigated the reactivity of a nucleophilic iron–oxygen oxidant, generated upon oxidative decarboxylation of an iron­(II)–benzilate complex [(Tp^Ph2)­Fe^II(benzilate)] (Tp^Ph2 = hydrotris­(3,5-diphenyl-pyrazol-1-yl)­borate), toward organoboronic acids. The oxidant converts different aryl/alkylboronic acids to the corresponding oxygenated products with the incorporation of one oxygen atom from dioxygen. This method represents an efficient protocol for the oxygenation of boronic acids with dioxygen as the terminal oxidant

Oxygenative Aromatic Ring Cleavage of 2‑Aminophenol with Dioxygen Catalyzed by a Nonheme Iron Complex: Catalytic Functional Model of 2‑Aminophenol Dioxygenases

2-Aminophenol dioxygenases catalyze the oxidative ring cleavage of 2-aminophenol to 2-picolinic acid using O₂ as the oxidant. Inspired by the reaction catalyzed by these nonheme iron enzymes, a biomimetic iron­(III)-2-amidophenolate complex, [(<i>t</i>Bu-L^Me)­Fe^III(4,6-di-<i>t</i>Bu-AP)]­(ClO₄) (<b>1a</b>) of a facial tridentate ligand (<i>t</i>Bu-L^Me = 1-[bis­(6-methyl-pyridin-2-yl)-methyl]-3-<i>tert</i>-butyl-urea and 4,6-di-<i>t</i>Bu-H₂AP = 2-amino-4,6-di-<i>tert</i>-butylphenol) bearing a urea group have been isolated. The complex reacts with O₂ to cleave the C–C bond of 4,6-di-<i>t</i>Bu-AP regioselectively and catalytically to afford 4,6-di-<i>tert</i>-butyl-2-picolinic acid. An iron­(II)-chloro complex [(<i>t</i>Bu-L^Me)­Fe^IICl₂(MeOH)] (<b>1</b>) of the same ligand also cleaves the aromatic ring of 4,6-di-<i>t</i>Bu-AP catalytically in the reaction with O₂. To assess the effect of urea group on the ring cleavage reaction of 2-aminophenol, two iron complexes, [(BA-L^Me)₂Fe^II₂Cl₄] (<b>2</b>) and [(BA-L^Me)­Fe^III(4,6-di-<i>t</i>Bu-AP)]­(ClO₄) (<b>2a</b>), of a tridentate ligand devoid of urea group (BA-L^Me = benzyl-[bis­(6-methyl-pyridin-2-yl)-methyl]-amine) have been isolated and characterized. Although the iron complexes (<b>1</b> and <b>1a</b>) of the ligand with urea group display catalytic reaction, the iron complexes (<b>2</b> and <b>2a</b>) of the ligand without urea group do not exhibit catalytic aromatic ring fission reactivity. The results support the role of urea group in directing the catalytic reactivity exhibited by <b>1</b> and <b>1a</b>

Iron(II)-catecholate complexes of a monoanionic facial <i style="">N₃</i> ligand: Structural and functional models of the extradiol cleaving catechol dioxygenases

420-426Two biomimetic iron(II)-catecholate complexes, [(TpPh2)FeII(CatH)] (1) and [(TpPh2)FeII(DBCH)] (2) (where TpPh2 = hydrotris(3,5-diphenylpyrazole-1-yl)borate, CatH = monoanionic pyrocatecholate and DBCH = monoanionic 3,5-di-tert-butyl catecholate), have been isolated and characterized to study their reactivity towards dioxygen. The single-crystal X-ray structure of (1) reveals a high-spin iron(II) center ligated by the monoanionic facial N3 ligand and a monoanionic catecholate, giving rise to a trigonal bipyramidal coordination geometry. Complex (1) represents the first structurally characterized five-coordinate iron(II)-catecholate complex with an asymmetric bidentate binding motif of monoanionic catecholate. While (1) reacts with dioxygen to form the corresponding iron(III)-catecholate, (2) reacts with dioxygen to give 75 % extradiol and 25 % intradiol cleavage products via an iron(III)-catecholate intermediate species. Complex (2) is a potential functional model of extradiol cleaving catechol dioxygenases

The methanol-methanolate CH3OH···CH3- bridging ligand: Tuning of exchange coupling by hydrogen bonds in dimethoxo-bridged dichromium(III) complexes

Two bis(mu-methoxo)dichromium(III) complexes, [(L2Cr2)-Cr- Se(mu-OCH3)(2)(CH3OH)(2)] 1 and [(L2Cr2)-Cr-Se(mu- OCH3)(2)(CH3OH)(CH3O)](-) 2, where L-Se represents the dianion of 2,2'-selenobis(4,6-di-tertbutylphenol), have been reported to demonstrate the effect of hydrogen bonding on the exchange coupling interactions between the chromium(Ill) centers. The corresponding sulfur analogue of the ligand, i.e., 2,2'- thiobis(4,6-di-tert-butylphenol), also yields the analogous [(L2Cr2)-Cr-S(Umu-OCH3)(2)(CH3OH)(2)] 3 and [(L2Cr2)-Cr-S(mu- OCH3)(2)(CH3O)(CH3OH)](-) 4, which exhibit similar exchange coupling parameters. An acid-base dependent equilibrium between I and 2 or 3 and 4 has been established by electronic spectral measurements

Reactivity of an Iron–Oxygen Oxidant Generated upon Oxidative Decarboxylation of Biomimetic Iron(II) α‑Hydroxy Acid Complexes

Three biomimetic iron­(II) α-hydroxy acid complexes, [(Tp^Ph2)­Fe^II(mandelate)­(H₂O)] (<b>1</b>), [(Tp^Ph2)­Fe^II(benzilate)] (<b>2</b>), and [(Tp^Ph2)­Fe^II(HMP)] (<b>3</b>), together with two iron­(II) α-methoxy acid complexes, [(Tp^Ph2)­Fe^II(MPA)] (<b>4</b>) and [(Tp^Ph2)­Fe^II(MMP)] (<b>5</b>) (where HMP = 2-hydroxy-2-methylpropanoate, MPA = 2-methoxy-2-phenylacetate, and MMP = 2-methoxy-2-methylpropanoate), of a facial tridentate ligand Tp^Ph2 [where Tp^Ph2 = hydrotris­(3,5-diphenylpyrazole-1-yl)­borate] were isolated and characterized to study the mechanism of dioxygen activation at the iron­(II) centers. Single-crystal X-ray structural analyses of <b>1</b>, <b>2</b>, and <b>5</b> were performed to assess the binding mode of an α-hydroxy/methoxy acid anion to the iron­(II) center. While the iron­(II) α-methoxy acid complexes are unreactive toward dioxygen, the iron­(II) α-hydroxy acid complexes undergo oxidative decarboxylation, implying the importance of the hydroxyl group in the activation of dioxygen. In the reaction with dioxygen, the iron­(II) α-hydroxy acid complexes form iron­(III) phenolate complexes of a modified ligand (Tp^Ph2*), where the ortho position of one of the phenyl rings of Tp^Ph2 gets hydroxylated. The iron­(II) mandelate complex (<b>1</b>), upon decarboxylation of mandelate, affords a mixture of benzaldehyde (67%), benzoic acid (20%), and benzyl alcohol (10%). On the other hand, complexes <b>2</b> and <b>3</b> react with dioxygen to form benzophenone and acetone, respectively. The intramolecular ligand hydroxylation gets inhibited in the presence of external intercepting agents. Reactions of <b>1</b> and <b>2</b> with dioxygen in the presence of an excess amount of alkenes result in the formation of the corresponding <i>cis</i>-diols in good yield. The incorporation of both oxygen atoms of dioxygen into the diol products is confirmed by ¹⁸O-labeling studies. On the basis of reactivity and mechanistic studies, the generation of a nucleophilic iron–oxygen intermediate upon decarboxylation of the coordinated α-hydroxy acids is proposed as the active oxidant. The novel iron–oxygen intermediate oxidizes various substrates like sulfide, fluorene, toluene, ethylbenzene, and benzaldehyde. The oxidant oxidizes benzaldehyde to benzoic acid and also participates in the Cannizzaro reaction